As a result of the trend toward weight reduction and size reduction in electrical appliances, attention is recently focused on a lithium secondary battery, which has a high energy density.
An electrolyte solution for a lithium secondary battery comprises a solute such as a lithium salt and an organic solvent. The organic solvent is required to have a high permittivity and a high oxidation potential and be stable in a battery. Since it is difficult for a single solvent to satisfy these requirements, a combination of a high-permittivity solvent such as, for example, a cyclic carbonic ester, e.g., ethylene carbonate or propylene carbonate, a cyclic carboxylic ester, e.g., γ-butyrolactone, or the like and a low-viscosity solvent such as a chain carbonic ester, e.g., diethyl carbonate or dimethyl carbonate, an ether, e.g., dimethoxyethane, or the like is used as the organic solvent of electrolyte solutions for a lithium secondary battery.
On the other hand, a lithium secondary battery is desired to be improved in various properties, and it has been proposed to contain various compounds in the electrolyte solution in order to meet the desire. Various phosphorus compounds have been investigated as such compounds to be contained in an electrolyte solution.
For example, to contain a phosphoric ester in an electrolyte solution is described in patent document 1 as a technique for improving suitability for trickle charge.
Furthermore, patent document 2 and patent document 3 describe a technique in which a specific phosphonic ester or phosphinic ester is containd into an organic solvent in an amount of from 5to 100% by weight to thereby impart fire retardancy to the electrolyte solution without adversely influencing battery performances. It is demonstrated in examples therein that in a secondary battery employing an electrolyte solution obtained by dissolving LiPF6 in an organic solvent prepared by mixing a carbonic ester or chain ether with those phosphoric esters in a weight ratio of 2:1 or 1:1, the decrease in capacity retention as measured at the 100th cycle is as small as from several percents to ten-odd percents. However, there is no statement therein concerning high-temperature characteristics of the battery.
Moreover, patent document 4 describes a photoregeneration type photoelectrochemical battery in which an ester such as phosphoric ester, an organic phosphonic ester, or an organic phosphinic ester is used, in place of carbonic ester or ether heretofore in use, as the solvent constituting the electrolyte solution to thereby prevent photo-electric converting characteristics from deteriorating with the lapse of time. There also is a statement to the effect that this electrolyte solution is applicable to a lithium-ion secondary battery. However, in this patent document 3 also, there is no statement concerning high-temperature characteristics of the secondary battery.
[Patent Document 1]
JP-A-11-233140
[Patent Document 2]
JP-A-10-228928
[Patent Document 3]
JP-A-11-233141
[Patent Document 4]
JP-A-2000-36332